This is a tutorial on the enumerator library, which is one implementation of Enumerator/Iteratee (EI) concept discovered by Oleg Kiselyov. The author of the library is John Millikin. EI is an API between a supplier (enumerator) and a consumer (iteratee). This API makes the following possible: Abstraction of data input sources If you implement a network code as iteratee, you can test it with just a